                addition, to sludge having a first pH value, of a basic agent based on lime, with a view to raising the pH to a second value higher than the first,        addition of at least one anionic organic flocculant, active at the said second pH value,        flocculation of the sludge, and        separation in the flocculated sludge between dewatered sludge and a liquid phase.        
This method concerns the treatment in particular of organic or oily sludge.
Within the meaning of the present invention, organic or oily sludge means all sludge excluding mineral sludge according to the classification set out on pages 119 to 123 of the work incorporated here by reference: Water Technical Memorandum, 9th èd, éd du Cinquantenaire, Rueil-Malmaison: Degrémont, 1989, 2 vol. Among these organic or oily sludges there is found for example purification station sludge from urban water and food industries. Other sludge, possibly more acid, can also be envisaged. The initial pH of the sludge is below 9, often below 8.
In general terms, the sludge is first of all settled after being thickened. It is then subjected to an aggregation step, referred to a coagulation and/or flocculation, followed by dewatering, that is to say solid/liquid separation, carried out in the majority of cases by means of a belt filter, a filter press or a centrifugal settler. Apart from the reduction in volume, the purpose of this treatment is to facilitate handling, transportation and storage of this sludge.
Incorporation of a limy compound, generally lime, is often associated with the aforementioned treatment, in order to sanitise and stabilise the sludge (for long-term storage thereof, holding in heaps, etc) and to durably improve its properties for use (pelletabilty, expandability, etc) or with a view to enhancing its agronomic value [Acta 5th European Biosolids and Organic Residuals Conference, Wakefield (UK), November 2000, paper 66].
Quick lime means a mineral solid material the chemical composition of which is principally calcium oxide CaO. Quick lime contains impurities, namely compounds such as magnesium oxide MgO, silica SiO2 or alumina Al2O3, etc, to the extent of a few percent. It should be understood that these impurities are expressed in the aforementioned forms but can in reality appear in different phases. In particular, what is expressed in the form of silica can in reality come for the major part from silicates.
Slaked lime or hydrated lime means a set of solid particles, mainly in the form of calcium hydroxide Ca(OH)2, obtained by “extinction”, sometimes referred to as “hydration”, of quick lime with water. This slaked lime can obviously contain the aforementioned impurities, issuing from quick lime.
Milk of lime means an aqueous suspension manufactured initially from quick lime or hydrated lime.
Organic and anionic flocculant means an anionic polymer having a molecular weight above 500,000 Da, preferably above 1,000,000 and more preferably above 5,000,000. Generally polymers are classified according to their length of chain, namely short-chain polymers and long-chain polymers. Typically short-chain polymers have an average molecular weight from a few thousands to a few tens of thousands of Da. Their size enables them to infiltrate between other molecules, which confers a dispersant character on them. Long-chain polymers have a molecular weight ranging from a few tens of thousands to a few millions of Da. The long chain enables them to “bridge” other molecules, which confers a flocculant character on them.
The lime can be added to the sludge before (pre-liming) or after (post-liming) the aforementioned dewatering step.
Apart from the fact that post-liming constitutes an additional mixing step in the sludge treatment process, this additional operation has several drawbacks, including                destructuring of the sludge,        the difficulty of mixing the lime with the dewatered sludge and, consequently        non-homogeneity of the distribution of the lime in the medium.        
All these reasons have led to the addition of lime by pre-liming, before dewatering, being envisaged.
The aforementioned aggregation is generally facilitated by the addition of a coagulating agent, including iron or aluminium salts, as proposed in the application WO 2006/030102. However, the use of these salts has many drawbacks, including the significant increase in the quantity of dry matter in the sludge, an attack on almost all metals, including stainless steels, because of their highly corrosive character, and danger for the operators, because of the irritant or even toxic character of the sludge given off by these salts during handling.
In order to overcome these drawbacks, it has become common practice to facilitate flocculation by means of organic compounds; in this case, the use of cationic flocculants is generally adapted to the treatment of organic sludge.
However, cationic polymers in general degrade rapidly from a pH of 9 to 10, as indicated in the prior art of the patent EP 1 154 958 B1. In many cases, the pre-liming operation can then not be carried out just before flocculation since the cationic polymer does not have time to act before the pH of the sludge reaches a critical value as from which deflocculation appears, because of the addition of lime. The aforementioned European patent EP 1154958 and the international patent application WO2005/014495, in particular, therefore proposed to resolve this problem by using specific lime or lime/magnesium compounds, which produce an increase in pH of the sludge that is sufficiently delayed for the cationic polymer to have the time to act as a flocculant before the solid/liquid separation. However, the aforementioned specific compounds are not suited to all cases of application.
This is because, in practice, the maintenance after liming, until the end of flocculation, of a pH at a value below that causing degradation of cationic organic flocculant will depend on the throughput of lime, the level of delayed basicity of the lime used, the proportion of dry matter and the buffering capacity of the liquid sludge. Given that, under industrial conditions, the proportion of dry matter and the buffering capacity of the sludge can fluctuate fairly greatly, the use of these processes using lime with delayed basicity sometimes appears tricky. Another disadvantage of the use of cationic organic flocculant in association with lime is the emission of volatile amines, an emission promoted at a high pH (CHANG, J.; ABU-ORF, M.; DENTEL, S., Alkylamine odors from degradation of flocculant polymers in sludges, Water Research, 2005, 39(14), pp. 3369-3375). Finally, the use of compounds delaying the rise in pH leads to an intentional extension of the combined flocculation and pre-liming step, not compatible with certain continuous dewatering processes.
Moreover, the application WO 09605142 proposes a method as indicated at the start. This method provides for a flocculation not of sludge but of a flow of waste water having a pH above 10.2, comprising an increase in its pH to a value above 10.2 if such is not the case initially, an addition of Mg ions and vigorous mixing with a highly anionic flocculant with a high molecular weight. The increase in pH can take place either through MgO, lime or sodium hydroxide. In a similar fashion, the application JP-54025268 describes a sludge flocculation method comprising the addition of an alkali NaOH or KOH, with a view to obtaining a pH above 10, followed by an addition of CaCl2 and next the addition of an anionic and/or non-ionic flocculant. These two methods require several steps, in particular an increase in the pH of the sludge beyond 10, and then the addition of Mg or CaCl2, prior to the addition of the anionic flocculant, which considerably extends the duration of the treatment of the sludge or reduces the flocculation time.
In the application JP-A-04-040286, an application to fishery sludge of an water-soluble calcium compound is provided and, before or after this application, addition of an organic flocculant, the latter being able to be cationic, anionic or non-ionic. In this case only the pH of the aqueous phase is taken into consideration, recommending neutralisation by an acid thereof after the addition of a basic calcium compound. In all the given examples, the sludge treated is initially highly basic and neutralisation always occurs after the application of the calcium compound and the addition of the organic flocculant. Use is made, as a calcium compound, either of highly basic compounds of the lime type or of calcium salts such as halides, nitrate or acetate. The objective sought is a purified water, resulting from the treatment of the sludge.